hydrogen bond

A weak chemical force that can operate both within and between certain types
of molecule. A hydrogen bond results when
a hydrogen atom that is attached to another
atom by a covalent bond is also attracted
to a neighboring electronegative
atom (such as oxygen or nitrogen),
either in the same molecule or in a separate molecule.

Although only about 10 percent as strong as covalent bonds, hydrogen bonds
are crucial to life as we know it. They are responsible for many of the
unique properties of water, for the tertiary
structure of proteins, and for the interaction
between purines and pyrimidines
which contributes to the stability of the DNA
double helix. Hydrogen bonding is also important in the hydrogen halides,
ice, alcohols, oxy-acids, ammonia,
amines, and amides.